1. Field of the Invention
This invention relates to aqueous radiation curable inkjet inks and to their use in ink jet printing.
2. Description of the Related Art
A latex is a stable colloidal dispersion of natural or synthetic polymer particles in an aqueous medium. Synthetic latexes have been made by polymerizing monomers, generally (meth)acrylates, that have been emulsified with surfactants. Such latexes have been incorporated in aqueous inkjet inks. For example, US 2008269410 A (HP) discloses self-dispersing acrylate latex particulates for ink-jet inks.
After jetting a latex inkjet ink, a latex film is formed as shown by FIG. 1. Acrylate based polymeric particles tend to have a relatively high minimum film formation temperature (MFT), which prevents to use thermosensitive substrates in latex inkjet printing. The MFT tends to be very close to the glass transition temperature Tg of the latex Thermosensitive substrates would be possible to use if the latex has a low Tg, e.g. below 0° C., however it was observed that such latexes cause jetting problems resulting in massive failing nozzles.
UV curable inkjet inks lacking water or organic solvents have been used extensively in industrial applications, because of their reliability and because that they adhere to a multitude of substrates. One disadvantage of these so-called ‘100% solids’ UV curable inkjet inks is that relatively thick layers are obtained compared to aqueous inkjet inks which generally contain less than 20% of solids. Thin layers usually have much better flexibility. However in most other properties aqueous latex inkjet inks tend to exhibit an inferior performance compared to UV curable inkjet inks.
Some efforts have been made to improve the performance of aqueous latex inkjet inks by including features of UV curable inkjet inks, such as the incorporation of water-soluble UV curable monomers, e.g. EP 2166046 A (FUJI) and US 2011104453 (CANON), the use of cross-linkable latexes as in EP 1379596 A (UCB), and the combination of both as in WO 2013/034880 A (SERICOL) which discloses a radiation-curable ink comprising: (a) a polyurethane having ethylenically unsaturated groups; (b) a water-soluble triacrylate; (c) a colorant; (d) a liquid medium comprising water and organic solvent; and (e) optionally a photoinitiator; and (f) optionally a surfactant; wherein the weight of component (d) is greater than the combined weight of components (a), (b), (c), (e) and (f). The latter does not show any results on jetting performance.
Therefore, there remains a need for aqueous inkjet inks containing polymeric particles capable of combining improved jetting performance with improved physical and chemical properties.